Cobalt based alloy product

ABSTRACT

There is provided a cobalt-based alloy product comprising: in mass %, 0.08-0.25% C; 0.1% or less B; 10-30% Cr; 5% or less Fe and 30% or less Ni, the total amount of Fe and Ni being 30% or less; W and/or Mo, the total amount of W and Mo being 5-12%; 0.5% or less Si; 0.5% or less Mn; 0.003-0.04% N; 0.5 to 2 mass % of an M component being a transition metal other than W and Mo and having an atomic radius of more than 130 pm; and the balance being Co and impurities. The impurities include 0.5% or less Al and 0.04% or less O. The product is a polycrystalline body of matrix phase crystal grains. In the matrix phase crystal grains, segregation cells with an average size of 0.13-2 μm are formed, in which the M component is segregated in boundary regions of the segregation cells.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to cobalt based alloy articles having excellent mechanical properties and, in particular, to a cobalt based alloy product applied with an additive manufacturing method.

DESCRIPTION OF BACKGROUND ART

Cobalt (Co) based alloy articles, along with nickel (Ni) based alloy articles, are representative heat resistant alloy materials. Also referred to as super alloys, they are widely used for high temperature members (components used under high temperature environment, e.g. gas turbine members, steam turbine members, etc.). Although Co based alloy articles are higher in material costs than Ni based alloy articles, they have been used for applications such as turbine stator blades and combustor members because of their excellence in corrosion resistance and abrasion resistance, and their ease of solid solution strengthening.

In Ni based alloy materials, various improvements that have been made so far in composition and manufacturing processes of heat resistant alloy materials have led to the development of strengthening through γ′ phase (e.g. Ni₃(Al, Ti) phase) precipitation, which has now become mainstream. On the other hand, in Co based alloy materials, an intermetallic compound phase that contributes to improving mechanical properties, like the γ′ phase in Ni based alloy materials, hardly precipitates, which has prompted research on carbide phase precipitation strengthening.

For example, Patent Literature 1 (JP Shou 61 (1986)-243143 A) discloses a Co based superplastic alloy made up of a Co based alloy matrix having a crystal grain size of equal to or less than 10 μm and carbide grains in a granular form or a particulate form having a grain size of 0.5 to 10 μm precipitated in the matrix. The Co based alloy includes 0.15 to 1 wt. % of C, 15 to 40 wt. % of Cr, 3 to 15 wt. % of W or Mo, 1 wt. % or less of B, 0 to 20 wt. % of Ni, 0 to 1.0 wt. % of Nb, 0 to 1.0 wt. % of Zr, 0 to 1.0 wt. % of Ta, 0 to 3 wt. % of Ti, 0 to 3 wt. % of Al, and the balance of Co. According to Patent Literature 1 (JP Shou 61 (1986)-243143 A), there can be provided a Co based superplastic alloy that exhibits superplasticity accompanying with an elongation of equal to or more than 70% even in a low temperature range (e.g. at 950° C.), and is capable of being formed into an article with a complicated shape by plastic working such as forging.

Patent Literature 2 (JP Hei 7 (1995)-179967 A) discloses a Co based alloy that is excellent in corrosion resistance, abrasion resistance, and high temperature strength. The alloy includes 21 to 29 wt. % of Cr, 15 to 24 wt. % of Mo, 0.5 to 2 wt. % of B, 0.1 or more and less than 0.5 wt. % of Si, more than 1 and equal to or less than 2 wt. % of C, 2 wt. % or less of Fe, wt. % or less of Ni, and the balance of substantially Co. According to Patent Literature 2 (JP Hei 7 (1995)-179967 A), the Co based alloy has a composite structure in which a molybdenum boride and a chromium carbide are relatively finely dispersed in a quaternary alloy phase of Co, Cr, Mo and Si and exhibits excellent corrosion resistance, abrasion resistance, and high strength.

Meanwhile, in recent years, three dimensional shaping technology (the so-called 3D printing) such as additive manufacturing or AM has received much attention as a technique for manufacturing finished products with a complicated shape by near net shaping. To apply the three dimensional shaping technology to heat resistant alloy components, vigorous research and development activities are currently being carried out.

For example, Patent Literature 3 (JP 2016-535169 A) discloses a method of producing layers including the steps of: (a) providing a source of a powdered or suspended granular composite powder having a porosity of 20% or less; (b) depositing a first portion of said powder onto a target surface; (c) depositing energy into the powder of said first portion under conditions that said energy causes sintering, fusing or melting of the first powder portion so as to create a first layer; (d) depositing a second portion of powder onto said first layer; and (e) depositing energy into the powder of said second portion under conditions that said energy causes sintering, fusing or melting of the second powder portion so as to create a second layer. In the method, the energy is supplied by a laser.

Patent Literature 3 (JP 2016-535169 A) teaches as follows: Selective laser melting (SLM) or direct metal laser melting (DMLM) uses laser to make a material a full melt. Full melting is typically useful for a monomaterial (e.g. pure titanium or a single alloy such as Ti-6Al-4V), as there is just one melting point. By contrast, selective laser sintering (SLS) and direct metal laser sintering (DMLS) are essentially the same thing, and SLS/DMLS is used to apply processes to a variety of materials—multiple metals, alloys, or combinations of alloys and other materials such as plastics, ceramics, polymers, carbides or glasses. Meanwhile, sintering is apart from melting, and a sintering process does not fully melt a material but heats it to the point that the material can fuse together on a molecular level.

CITATION LIST Patent Literature

Patent Literature 1: JP Shou 61 (1986)-243143 A,

Patent Literature 2: JP Hei 7 (1995)-179967 A, and

Patent Literature 3: JP 2016-535169 A.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Since the 3D printing is capable of directly forming even components of complicated shape, manufacturing of turbine high temperature components by the 3D printing is very attractive in terms of reduction of manufacturing work time and improvement of manufacturing yield (i.e. reduction of manufacturing cost).

Co based alloy materials such as the ones disclosed in Patent Literatures 1 and 2 are thought to have mechanical properties superior to those of previous Co based alloy materials. Unfortunately, however, their mechanical properties are inferior to those of precipitation-strengthened Ni based alloy materials of recent years. Therefore, many studies on additively manufactured articles (AM articles) for use as high temperature components are currently directed toward precipitation-strengthened Ni based alloy materials.

However, AM articles of the precipitation-strengthened Ni based alloys are prone to have problems such as generation of the γ′ phase, which is the core of their mechanical properties, being hindered and internal defects occurring in the finished products. As a result, expected mechanical properties have not been sufficiently achieved. This is attributable to the fact that current precipitation-strengthened Ni based alloy materials used for high temperature components have been optimized through melting and forging processes under high vacuum, and therefore oxidation and nitriding of the Al component and the Ti component, which constitute the γ′ phase, easily occur at the stages of preparing alloy powder for AM and performing AM.

On the other hand, manufacturing the Co based alloy materials such as the ones disclosed in Patent Literatures 1 and does not require precipitation of an intermetallic compound phase such as the γ′ phase as in Ni based alloy materials, so Co based alloy materials do not contain plenty of Al or Ti, which is easily oxidized. This means melting and forging processes in the air atmosphere are available for their manufacturing. Therefore, such Co based alloy materials are considered to be advantageous in manufacturing of alloy powder for AM and manufacturing of AM articles. Also, the Co based alloy materials have advantages with corrosion resistance and abrasion resistance comparable to or superior to those of Ni based alloy materials.

However, as mentioned above, conventional Co based alloy materials have disadvantages of mechanical properties inferior to those of γ′ phase precipitation-strengthened Ni based alloy materials. In other words, if a Co based alloy material could achieve mechanical properties comparable to or superior to those of γ′ phase precipitation-strengthened Ni based alloy materials (e.g. a creep rupture time of 1,100 hours or more by a creep test under conditions of a temperature of 900° C. and a stress of 98 MPa), AM articles of the Co based alloy material would become highly attractive high temperature components.

The present invention was made in view of the foregoing and has an objective to provide a Co based alloy product having mechanical properties comparable to or superior to those of precipitation strengthened Ni based alloy materials.

Solution to Problems

(I) According to one aspect of the present invention, there is provided a cobalt based alloy product. The cobalt based alloy product has a chemical composition including: 0.08 to 0.25 mass % of carbon (C); 0.1 mass % or less of boron (B); 10 to 30 mass % of chromium (Cr); 5 mass % or less of iron (Fe) and 30 mass % or less of nickel (Ni), the total amount of the Fe and the Ni being 30 mass % or less; tungsten (W) and/or molybdenum (Mo), the total amount of the W and the Mo being 5 to 12 mass %; 0.5 mass % or less of silicon (Si); 0.5 mass % or less of manganese (Mn); 0.003 to 0.04 mass % of nitrogen (N); 0.5 to 2 mass % of an M component being a transition metal other than W and Mo and having an atomic radius of more than 130 pm; and the balance being cobalt (Co) and impurities. The impurities include 0.5 mass % or less of aluminum (Al), and 0.04 mass % or less of oxygen (O). The product is a polycrystalline body of matrix phase crystal grains. In the matrix phase crystal grains of the polycrystalline body, segregation cells with an average size of 0.13 to 2 μm are formed, in which the M component is segregated in boundary regions of the segregation cells.

In the above Co based alloy product (I) of the invention, the following changes and modifications can be made.

(i) The M component may be a component being capable of generating an MC type carbide phase of a simple cubic crystal system; and grains of the MC type carbide phase may be precipitated on the boundary regions of the segregation cells.

(ii) The M component of the chemical composition may be at least one of titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb) and tantalum (Ta).

(iii) In the case that the chemical composition includes the Ti as the M component, content of the Ti may be 0.01 to 1 mass %; in the case that the chemical composition includes the Zr as the M component, content of the Zr may be 0.05 to 1.5 mass %; in the case that the chemical composition includes the Hf as the M component, content of the Hf may be 0.01 to 0.5 mass %, in the case that the chemical composition includes the V as the M component, content of the V may be 0.01 to 0.5 mass %; in the case that the chemical composition includes the Nb as the M component, content of the Nb may be 0.02 to 1 mass %; and in the case that the chemical composition includes the Ta as the M component, content of the Ta may be 0.05 to 1.5 mass %.

(iv) The Zr may be an essential component as the M component of the chemical composition.

(v) The product may exhibit a creep rupture time of 1,500 hours or more by a creep test under conditions of a temperature of 850° C. and a stress of 180 MPa.

(vi) The product may be a high temperature member.

(vii) The high temperature member may be a turbine stator blade, a turbine rotor blade, a turbine combustor nozzle, or a heat exchanger.

Advantages of the Invention

According to the present invention, there can be provided a Co based alloy product having mechanical properties comparable to or superior to those of precipitation strengthened Ni based alloy materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram showing an exemplary process of a method for manufacturing a Co based alloy product according to an embodiment of the present invention;

FIG. 2 is a scanning electron microscope (SEM) image showing an exemplary microstructure of a Co based alloy product according to an embodiment of the invention;

FIG. 3 is an SEM image showing another exemplary microstructure of a Co based alloy product according to an embodiment of the invention;

FIG. 4 is a schematic illustration of a perspective view showing a turbine stator blade which is a Co based alloy product as a high temperature member according to an embodiment of the invention;

FIG. 5 is a schematic illustration of a cross-sectional view showing a gas turbine equipped with a Co based alloy product according to an embodiment of the invention;

FIG. 6 is a schematic illustration of a perspective view showing a heat exchanger which is a Co based alloy product as a high temperature member according to an embodiment of the invention; and

FIG. 7 shows exemplary selective laser melting (SLM) conditions in the SLM step, indicating a relationship between a thickness of an alloy powder bed and a local heat input.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Basic Concept of the Present Invention]

As mentioned before, various research and development activities have been carried out on strengthening of Co based alloy materials by means of precipitation of a carbide phase of a transition metal. Examples of the carbide phase that can be precipitated include MC type, M₂C type, M₃C type, M₆C type, M₇C type, and M₂₃C₆ type carbide phases.

In the conventional Co based alloy materials, grains of the carbide phases often precipitate along final solidification portions (e.g. dendrite boundaries, crystal grain boundaries, etc. of the matrix phase) at the casting stages of the Co based alloys. In a general cast material of a Co based alloy, for example, the average spacing between dendrite boundaries and the average crystal grain size are on the order of 10¹ to 10² μm, and therefore the average spacing between carbide phase grains is also on the order of 10¹ to 10² μm. Furthermore, even with the relatively fast solidification rate of laser welding, for example, the average spacing between carbide phase grains at the solidified portions is around 5 μm.

Precipitation strengthening in alloys is generally known to be inversely proportional to the average spacing between precipitates, and it is considered that precipitation strengthening is effective only when the average spacing between precipitates is around 2 μm or less. However, with the above-mentioned conventional technology, the average spacing between precipitates has not reached this level in a Co based alloy material, and sufficient precipitation strengthening effect has not been achieved. In other words, with the conventional technology, it has been difficult to finely and dispersedly precipitate carbide phase grains that might contribute to strengthening alloys. This would be the main factor behind the fact that Co based alloy materials have been said to have mechanical properties inferior to those of precipitation-strengthened Ni based alloy materials.

The present inventors thought that if they were able to dispersedly precipitate carbide phase grains contributing to precipitation strengthening in the matrix phase crystal grains, they would be able to dramatically improve mechanical properties of Co based alloy materials. Considering the inherent excellent corrosion resistance and abrasion resistance of Co based alloy materials, they would be able to provide a heat-resistant alloy material that would surpass precipitation-strengthened Ni based alloy materials.

Then, in order to obtain such a Co based alloy material, the inventors conducted intensive research on alloy compositions and manufacturing methods. In the initial stages of the research, the inventors had considered that it would be effective to generate a carbide phase of a metal element that is highly solid soluble and hardly segregate in the Co based alloy matrix phase, so that the carbide phase was easy to precipitate dispersedly in the matrix phase crystal grains. Unfortunately, however, precipitation of the Cr carbide phase (in this case, Cr₂₃C₆ phase) was not so effective to strengthen the Co based alloy material. Then, as an idea of reversal, the inventors considered to intentionally form carbides of metal elements having tendency to segregate at the solidification stages of the Co based alloys.

The inventors focused on the atomic radii of metal elements constituting the Co based alloys.

For example, the Co based alloy disclosed in Patent Literature 1 includes, as a metal element, Co (atomic radius: 125 pm), Cr (atomic radius: 128 pm), Ni (atomic radius: 124 pm), W (atomic radius: 139 pm), Mo (atomic radius: 139 pm), Nb (atomic radius: 146 pm), Zr (atomic radius: 160 pm), Ta (atomic radius: 146 pm), Ti (atomic radius: 147 pm), and Al (atomic radius: 143 pm). In the Co based alloy of Patent Literature 1, since more than 70% by mass is made of Co, Cr and Ni, the atomic radius of most constituent atoms of the alloy is 130 pm or less. In other words, it was considered that each of W, Mo, Nb, Zr, Ta, Ti and Al having the atomic radius of more than 130 pm might be easy to segregate at the solidification stage of the Co based alloy. And then, the inventors carried out the research on a method for dispersedly precipitating the carbide phases of these transition metals except Al in the matrix phase crystal grains.

As a result, they have found that segregation cells with a small size are formed, in which specific components (components forming carbide phases contributing to alloy strengthening) are segregated, in the matrix phase crystal grains of a Co based alloy additively manufactured article by optimizing the alloy composition and controlling the amount of heat input for local melting and rapid solidification in an additive manufacturing method (in particular, selective laser melting). Also, they have found that grains of precipitation reinforcing carbide phase may be dispersedly precipitated on a part of boundary regions of the segregation cells. The present invention was made based on these findings.

Preferred embodiments of the present invention will be hereinafter described with reference to the accompanying drawings. However, the invention is not limited to the specific embodiments described below, and various combinations with known art and modifications based on known art are possible without departing from the spirit and the scope of the invention.

[Method for Manufacturing Co Based Alloy Product]

FIG. 1 is a flow diagram showing an exemplary process of a method for manufacturing a Co based alloy product according to an embodiment of the invention. As shown in FIG. 1, the method for manufacturing a Co based alloy product roughly includes: an alloy powder preparation step S1 of preparing a Co based alloy powder; and a selective laser melting step S2 of forming the prepared Co based alloy powder into an AM article with a desired shape. The method for manufacturing a Co based alloy product may further include an internal strain relaxation annealing step S3 of subjecting the Co based alloy product to an annealing for relaxing a residual internal strain of the product.

Furthermore, a Co based alloy product of the invention may be an article that is applied a corrosion resistant coating layer formation step and/or a surface finishing step, not shown in FIG. 1, to the article obtained by the step S2 or the step S3.

Each step will be hereinafter described in more detail.

(Alloy Powder Preparation Step)

In the step S1, a Co based alloy powder having a predetermined chemical composition is prepared. The chemical composition preferably includes: 0.08 to 0.25 mass % of C; 0.1 mass % or less of B; 10 to 30 mass % of Cr; 5 mass % or less of Fe and 30 mass % or less of Ni, the total amount of the Fe and the Ni being 30 mass % or less; W and/or Mo, the total amount of the W and the Mo being 5 to 12 mass %; 0.5 mass % or less of Si; 0.5 mass % or less of Mn; 0.003 to 0.04 mass % of N; 0.5 to 2 mass % of an M component being a transition metal other than W and Mo and having an atomic radius of more than 130 pm; and the balance being Co and impurities. As impurities, 0.5 mass % or less of Al and 0.04 mass % or less of O may be included.

C: 0.08 to 0.25 mass %

The C component is an important component that constitutes an MC type carbide phase of a simple cubic crystal system to serve as a precipitation strengthening phase (this MC type carbide phase may be referred to as “carbide phase of Ti, Zr, Hf, V, Nb and/or Ta” or “precipitation reinforcing carbide phase”). The content of the C component is preferably 0.08 to 0.25 mass %, more preferably 0.1 to 0.2 mass %, and even more preferably 0.12 to 0.18 mass %. When the C content is less than 0.08 mass %, the amount of precipitation of the precipitation reinforcing carbide phase is insufficient, resulting in an insufficient effect of improving the mechanical properties. By contrast, when the C content is over 0.25 mass %, carbide phases other than the MC type carbide phase precipitate excessively, and/or the alloy material becomes excessively hard, which leads to deteriorated ductility and toughness.

B: 0.1 mass % or less

The B component contributes to improving bondability between crystal grain boundaries (the so-called grain boundary strengthening). Although the B is not an essential component, when it is contained in the alloy, the content of the B component is preferably 0.1 mass % or less and more preferably 0.005 to 0.05 mass %. When the B component is over 0.1 mass %, cracking (e.g. solidification cracking) is prone to occur during formation of the AM article.

Cr: 10 to 30 mass %

The Cr component contributes to improving corrosion resistance and oxidation resistance. The content of the Cr component is preferably 10 to 30 mass % and more preferably 15 to 27 mass %. In the case where a corrosion resistant coating layer is provided on the outermost surface of the Co based alloy product, the content of the Cr component is even more preferably 10 to 18 mass %. When the Cr content is less than 10 mass %, advantageous effects such as improvements of the corrosion resistance and the oxidation resistance are insufficient. When the Cr content is over 30 mass %, the brittle σ phase and/or the excessive amount of Cr carbide phase are generated, resulting in deteriorated mechanical properties (i.e. toughness, ductility, strength, etc.). Meanwhile, in the invention Cr carbide phase generation itself in the article is not denied.

Ni: 30 mass % or less

Being similar to Co component in properties but less expensive than Co, the Ni component may be used to replace part of the Co component. Although the Ni is not an essential component, when it is contained in the alloy, the content of the Ni component is preferably 30 mass % or less, more preferably 20 mass % or less, and more preferably 5 to 15 mass %. When the Ni content is over 30 mass %, the abrasion resistance and the local stress resistance, which are characteristics of Co based alloys, deteriorate. This is attributable to the difference in stacking fault energy between Co and Ni.

Fe: 5 mass % or less

Being much less expensive than Ni and similar to Ni component in properties, the Fe component may be used to replace part of the Ni component. The total content of the Fe and Ni is preferably 30 mass % or less, more preferably 20 mass % or less, and even more preferably 5 to 15 mass %. Although the Fe is not an essential component, when it is contained in the alloy, the content of the Fe component is preferably 5 mass % or less and more preferably 3 mass % or less in the range less than the Ni content. When the Fe content is over 5 mass %, the corrosion resistance and mechanical properties deteriorate.

W and/or Mo: 5 to 12 mass % in total

The W component and the Mo component contribute to solution-strengthening the matrix. The total content of the W component and/or the Mo component (at least one of W and Mo components) is preferably 5 to 12 mass % and more preferably 7 to 10 mass %. When the total content of the W component and the Mo component is less than 5 mass %, the solution strengthening of the matrix is insufficient. In contrast, when the total content of the W component and the Mo component is over 12 mass %, the brittle σ phase tends to be generated easily, resulting in deteriorated mechanical properties (i.e. toughness, ductility, etc.).

Re: 2 mass % or less

The Re component contributes to solution-strengthening the matrix and improving corrosion resistance. Although the Re is not an essential component, when it is contained in the alloy to replace part of the W component or the Mo component, the content of the Re component is preferably 2 mass % or less and more preferably 0.5 to 1.5 mass %. When the Re content is over 2 mass %, the advantageous effects of the Re component become saturated, and the material costs become too high.

M component of a transition metal other than W and Mo and with an atomic radius of more than 130 pm: 0.5 to 2 mass % in total

As an M component being a transition metal other than W and Mo, having an atomic radius of more than 130 pm, and being capable to forming an MC type carbide phase of a simple cubic crystal system, there can be listed Ti, Zr, Hf, V, Nb and Ta components. The MC type carbide phase could become a precipitation reinforcing carbide phase for the product of the invention. In other words, it is preferable that at least one of the Ti, Zr, Hf, V, Nb and Ta components is included. The total content of the Ti, Zr, Hf, V, Nb and Ta components is preferably 0.5 to 2 mass % and more preferably 0.5 to 1.8 mass %. When the total content is less than 0.5 mass %, the amount of precipitation of the precipitation reinforcing carbide phase is insufficient, and, as a result, the effect of improving the mechanical properties is insufficient. In contrast, when the total content is over 2 mass %, the mechanical properties deteriorate due to coarsening of the grains of the precipitation reinforcing carbide phase, accelerated generation of a brittle phase (e.g. σ phase), generation of grains of an oxide phase that does not contribute to precipitation strengthening, etc.

More specifically, when the Ti component is included, the Ti content is preferably 0.01 to 1 mass % and more preferably 0.05 to 0.8 mass %.

When the Zr component is included, the Zr content is preferably 0.05 to 1.5 mass % and more preferably 0.1 to 1.2 mass %. From the viewpoint of the mechanical strength, it is preferable that the Zr component is included in the alloy.

When the Hf component is included, the Hf content is preferably 0.01 to 0.5 mass % and more preferably 0.02 to 0.1 mass %.

When the V component is included, the V content is preferably 0.01 to 0.5 mass % and more preferably 0.02 to 0.1 mass %.

When the Nb component is included, the Nb content is preferably 0.02 to 1 mass % and more preferably 0.05 to 0.8 mass %.

When the Ta component is included, the Ta content is preferably 0.05 to 1.5 mass % and more preferably 0.1 to 1.2 mass %.

Si: 0.5 mass % or less

The Si component serves as a deoxidant agent and contributes to improving the mechanical properties. Although the Si is not an essential component, when it is contained in the alloy, the content of the Si component is preferably 0.5 mass % or less and more preferably 0.01 to 0.3 mass %. When the Si content is over 0.5 mass %, coarse grains of an oxide (e.g. SiO₂) are generated, which causes deterioration of the mechanical properties.

Mn: 0.5 mass % or less

The Mn component serves as a deoxidant agent and a desulfurizing agent and contributes to improving the mechanical properties and the corrosion resistance. Although the Mn is not an essential component, when it is contained in the alloy, the content of the Mn component is preferably 0.5 mass % or less and more preferably 0.01 to 0.3 mass %. When the Mn content is over 0.5 mass %, coarse grains of a sulfide (e.g. MnS) are generated, which causes deterioration of the mechanical properties and the corrosion resistance.

N: 0.003 to 0.04 mass % The N component contributes to stabilizing the generation of the precipitation reinforcing carbide phase. The content of the N component is preferably 0.003 to 0.04 mass %, more preferably 0.005 to 0.03 mass %, and even more preferably 0.007 to 0.025 mass %. When the N content is less than 0.003 mass %, the advantageous effects of the N component are insufficient. Meanwhile, when the N content is over 0.04 mass %, coarse grains of a nitride (e.g. Cr nitride) are generated, which causes deterioration of the mechanical properties.

Balance: Co component and Impurities

The Co component is one of the key components of the alloy and its content is the largest of all the components. As mentioned above, Co based alloy materials have the advantages of having corrosion resistance and abrasion resistance comparable to or superior to those of Ni based alloy materials.

The Al component is one of the impurities of the alloy and is not to be intentionally included in the alloy. However, an Al content of 0.5 mass % or less is acceptable as it does not have any serious negative influence on the mechanical properties of the Co based alloy product. When the Al content is over 0.5 mass %, coarse grains of an oxide or nitride (e.g. Al₂O₃ or AlN) are generated, which causes deterioration of the mechanical properties.

The O component is also one of the impurities of the alloy and is not to be intentionally included in the alloy. However, an O content of 0.04 mass % or less is acceptable as it does not have any serious negative influence on the mechanical properties of the Co based alloy product. When the O content is over 0.04 mass %, coarse grains of each oxide (e.g. Ti oxide, Zr oxide, Al oxide, Fe oxide, Si oxide, etc.) are generated, which causes deterioration of the mechanical properties.

In this step S1, there is no particular limitation on the method and techniques for preparing the Co based alloy powder, and any conventional method and technique may be used. For example, a master ingot manufacturing substep S1 a of manufacturing a master ingot by mixing, melting, and casting the raw materials such that the ingot has a desired chemical composition and an atomization substep S1 b of forming the alloy powder from the master ingot may be performed. Also, there is no particular limitation on the atomization method, and any conventional method and technique may be used. For example, gas atomizing or centrifugal force atomizing, by which spherical particles of high purity can be obtained, may be preferably used.

For ease of handling and ease of filling the alloy powder bed in the following selective laser melting step S2, the particle size of the alloy powder is preferably 5 to 100 μm, more preferably 10 to 70 μm, and even more preferably 10 to 50 μm. When the particle size of the alloy powder is less than 5 μm, fluidity of the alloy powder decreases in the following step S2 (i.e. formability of the alloy powder bed decreases), which causes deterioration of shape accuracy of the AM article. In contrast, when the particle size of the alloy powder is over 100 μm, controlling the local melting and rapid solidification of the alloy powder bed in the following step S2 becomes difficult, which leads to insufficient melting of the alloy powder and increased surface roughness of the AM article.

In view of the above, an alloy powder classification substep S1 c is preferably performed so as to regulate the alloy powder particle size to 5 to 100 μm. In the present invention, when the particle size distribution of the alloy powder manufactured in the atomization substep S1 b is observed to fall within the desired range, it is assumed that the substep S1 c has been performed.

(Selective Laser Melting Step)

In the step S2, the prepared Co based alloy powder is formed into an AM article having a desired shape by selective laser melting (SLM). More specifically, this step comprises alternate repetition of an alloy powder bed preparation substep S2 a and a laser melting solidification substep S2 b. In the step S2 a, the Co based alloy powder is laid such that it forms an alloy powder bed having a predetermined thickness, and in the step S2 b, a predetermined region of the alloy powder bed is irradiated with a laser beam to locally melt and rapidly solidify the Co based alloy powder in the region.

In this step S2, in order to obtain a finished Co based alloy product having a desired microstructure (a microstructure in which the segregation cells are formed in the matrix phase crystal grains, and components constituting the MC type carbide phase are segregated in boundary regions of the segregation cells; and/or a microstructure in which the precipitation reinforcing carbide phase grains are dispersedly precipitated in the matrix phase crystal grains), the microstructure of the AM article, which is a precursor of the finished product, is controlled by controlling the local melting and the rapid solidification of the alloy powder bed.

More specifically, the thickness of the alloy powder bed h (unit: μm), the output power of the laser beam P (unit: W), and the scanning speed of the laser beam S (unit: mm/s) are preferably controlled to satisfy the following formulas: “15<h<150” and “67×(P/S)−3.5<h<2222×(P/S)+13”. When these formulas are not satisfied, an AM article having a desired microstructure cannot be obtained.

While the output power P and the scanning speed S of the laser beam basically depend on configurations of the laser apparatus, they may be determined so as to satisfy the following formulas: “10≤P≤1000” and “10≤S≤7000”.

(Internal Strain Relaxation Annealing Step) As described before, an internal strain relaxation annealing step S3 may be applied to the product obtained by the SLM step S2. As an internal strain relaxation annealing, it is preferable to conduct a heat treatment at, e.g., 400° C. or more and less than 600° C. Although this step S3 is not an essential step, residual internal strain of the product possibly caused by rapid solidification in the SLM step S2 can be relaxed, and undesired deformation of the alloy product can be restrained during using as a high temperature member.

(Co Based Alloy Product)

FIG. 2 is a scanning electron microscope (SEM) image showing an exemplary microstructure of a Co based alloy product of the invention; and FIG. 3 is an SEM image showing another exemplary microstructure of a Co based alloy product of the invention. As shown in FIGS. 2 and 3, the Co based alloy products obtained by the SLM step S2 have a unique microstructure that has never been seen before.

The AM article is a polycrystalline body of matrix phase crystal grains. In the matrix phase crystal grains of the polycrystalline body, segregation cells with an average size of 0.13 to 2 μm are formed. In the viewpoint of the mechanical strength, segregation cells with an average size of 0.15 to 1.5 μm are more preferable. It may be recognized that grains of the carbide phase are precipitated on a part of boundary regions of the segregation cells. In addition, from various experiments by the inventors, it can be recognized that the matrix phase crystal grains with an average size of 5 to 150 μm are preferable.

In the present invention, the size of segregation cells is basically defined as the average of the long diameter and the short diameter. However, when an aspect ratio of the longer diameter and the short diameter is three or more, twice the short diameter may be adopted as the size of segregation cell.

A more detailed microstructure observation by scanning transmission electron microscopy-energy dispersive X-ray spectrometry (STEM-EDX) has revealed that the components constituting the MC type carbide phase (Ti, Zr, Hf, V, Nb, Ta, and C) segregate in the boundary regions between the neighboring segregation cells (i.e. in outer peripheral regions of micro-cells, similar to cell walls). It has also been observed that grains precipitating on the boundary regions among these segregation cells are the MC type carbide phase grains.

Naturally enough, the segregation of the M components constituting the MC type carbide phase and the precipitation of the MC type carbide phase grains can be observed also on the grain boundaries of the matrix phase crystal grains in the Co based alloy product of the invention.

FIG. 4 is a schematic illustration of a perspective view showing a turbine stator blade which is a Co based alloy product as a high temperature member according to an embodiment of the invention. As shown in FIG. 4, the turbine stator blade 100 includes an inner ring side end wall 101, a blade part 102, and an outer ring side end wall 103. Inside the blade part 102 is often formed a cooling structure. As seen from FIG. 4, since the turbine stator blade 100 has a very complicated shape and structure, the technical significance of alloy products based on the AM articles manufactured by near net shaping is profound.

Meanwhile, in the case of a gas turbine for power generation with an output of around 30 MW, the length of the blade part 102 of the turbine stator blade 100 (i.e. distance between the end walls 101 and 103) is approximately 170 mm. Naturally enough, a Co based alloy product of the invention can be used as a turbine rotor blade.

FIG. 5 is a schematic illustration of a cross-sectional view showing a gas turbine equipped with a Co based alloy product according to an embodiment of the invention. As shown in FIG. 5, the gas turbine 200 roughly includes a compression part 210 for compressing intake air and a turbine part 220 for blowing combustion gas of a fuel on turbine blades to obtain rotation power. The high temperature member according to the embodiment of the invention can be preferably used as a turbine nozzle 221 or the turbine stator blade 100 inside the turbine part 220. The high temperature member according to the embodiment of the invention is not limited to gas turbine applications but may be used for other turbine applications (e.g. steam turbines) and component used under high temperature environment in other machines/apparatuses.

FIG. 6 is a schematic illustration of a perspective view showing a heat exchanger which is a Co based alloy product as a high temperature member according to an embodiment of the invention. A heat exchanger 300 shown in FIG. 6 is an example of a plate-fin type heat exchanger, and has a basic structure in which a separation layer 301 and a fin layer 302 are alternatively stacked each other. Both ends in the width direction of flow channels in the fin layer 302 are sealed by a side bar portion 303. Heat exchanging between high temperature fluid and low temperature fluid can be done by flowing the high temperature fluid and the low temperature fluid alternately into adjacent fin layers 302 via the separation layer 301.

A heat exchanger 300 according to an embodiment of the invention is formed integrally without soldering joining or welding joining the conventional parts constituting a heat exchanger such as separation plates, corrugated fins and side bars. Consequently, the heat exchanger 300 has advantages improving heat resistance and weight reduction than the conventional heat exchangers. In addition, the heat transfer efficiency can be higher by forming an appropriate concavo-convex pattern on the surfaces of the flow channels and making the fluid into turbulence. Improving the heat transfer efficiency leads to downsizing of the heat exchanger.

EXAMPLES

The present invention will be hereinafter described in more detail with examples and comparative examples. It should be noted that the invention is not limited to these examples.

(Preparation of Alloy Powders IA-1 to IA-7 and CA-1 to CA-5) Co based alloy powders having the chemical compositions shown in Table 1 were prepared (the alloy powder preparation Step S1). Specifically, first, the master ingot manufacturing substep S1 a was performed, in which the raw materials were mixed and subjected to melting and casting by a vacuum high frequency induction melting method so as to form a master ingot (weight: approximately 2 kg) for each powder. Next, the atomization substep S1 b was performed to form each alloy powder. In the substep S1 b, each master ingot was remelted and subjected to gas atomizing in an argon gas atmosphere.

Then, each alloy powder thus obtained was subjected to the alloy powder classification substep S1 c to control the particle size of alloy powder. Each alloy powder was classified into an alloy powder with a particle size of 5 to 25 μm.

TABLE 1 Chemical Compositions of Alloy Powders IA-1 to IA-7 and CA-1 to CA-5. Chemical Composition (mass %) Ti + Zr + Alloy Hf + V + Powder C B Cr Ni Fe W Ti Zr Hf V Nb Ta Si Mn N Co Al O Nb + Ta IA-1 0.16 0.009 24.7 9.3 0.01 7.5 0.16 0.45 — — 0.20 0.15 0.01 0.01 0.005 Bal. 0.01 0.005 0.96 IA-2 0.25 0.011 26.5 10.5 0.90 7.4 0.30 0.60 — — 0.15 0.40 0.30 0.20 0.030 Bal. 0.05 0.020 1.45 IA-3 0.08 0.009 30.0 — — 5.0 — 0.35 — — 0.16 — 0.05 0.01 0.005 Bal. — 0.005 0.51 IA-4 0.10 0.010 25.0 8.0 0.02 7.5 0.25 0.05 — — 0.09 0.30 0.01 0.02 0.010 Bal. — 0.010 0.69 IA-5 0.18 0.009 24.9 9.2 0.01 7.6 0.17 0.45 0.02 0.04 0.21 0.16 0.01 0.01 0.015 Bal. 0.01 0.010 1.05 IA-6 0.24 0.011 25.5 10.3 0.90 7.4 0.20 0.60 0.05 0.02 0.15 0.40 0.30 0.20 0.040 Bal. 0.06 0.025 1.42 IA-7 0.08 0.009 29.5 — — 6.0 0.10 0.15 0.01 0.04 — 0.30 0.15 0.10 0.005 Bal. — 0.005 0.60 CA-1 0.35 0.009 32.5 9.5 0.01 7.3 0.15 0.40 — — 0.05 0.50 0.01 0.01 0.005 Bal. 0.01 0.005 1.10 CA-2 0.35 0.009 30.0 40.0 0.01 7.3 0.90 0.40 — — 1.0  1.0  0.01 0.01 0.005 Bal. 2.20 0.005 3.30 CA-3 0.40 0.010 29.0 10.0 0.20 7.5 0.20 0.10 — — 0.10 — 0.10 0.02 0.001 Bal. — 0.015 0.40 CA-4 0.25 0.010 29.0 10.0 0.10 7.0 — — — — — — — 0.01 0.010 Bal. — 0.010 0 CA-5 0.11 0.002 22.0 23.0 0.01 14.0 0.01 0.01 — — — — 0.50 0.003 0.006 Bal. 0.01 0.008 0.02 “—” indicates that the element was not intentionally included or not detected. “Bal.” indicates inclusion of impurities other than Al and O.

As shown in Table 1, the inventive alloy powders IA-1 to IA-7 have chemical compositions that satisfy the specifications of the invention. In contrast, the comparative alloy powder CA-1 has a C content and a Cr content that fail to satisfy the specifications of the invention. The comparative alloy powder CA-2 has a C content, an Ni content, and a total content of “Ti+Zr+Hf+V+Nb+Ta” that are out of the specifications of the invention. The comparative alloy powder CA-3 has a C content, an N content and a total content of “Ti+Zr+Hf+V+Nb+Ta” that are outside of the specifications of the invention. The comparative alloy powder CA-4 has a total content of “Ti+Zr+Hf+V+Nb+Ta” that fail to satisfy the specifications of the invention. The comparative alloy powder CA-5 has a W content and a total content of “Ti+Zr+Hf+V+Nb+Ta” that are out of the specifications of the invention.

[Experiment 2]

(Examination of SLM Conditions in Selective Laser Melting Step)

AM articles (8 mm in diameter×10 mm in length) were formed of the alloy powders IA-4 prepared in Experimental 1 by the SLM process (selective laser melting step S2). The output power of the laser beam P was set at 85 W, and the local heat input P/S (unit: W×s/mm=J/mm) was controlled by varying the thickness of the alloy powder bed h and the scanning speed (mm/s) of the laser beam S. Controlling the local heat input corresponds to controlling the cooling rate.

The AM articles formed above were each subjected to microstructure observation to measure the average segregation cell size. The microstructure observation was performed by SEM. Also, the obtained SEM images were subjected to image analysis using an image processing program (ImageJ, a public domain program developed at the National Institutes of Health in U.S.A., or NIH) to measure the average size of segregation cells.

FIG. 7 shows exemplary SLM conditions in the SLM step S2, indicating a relationship between the thickness of the alloy powder bed and the local heat input. In FIG. 7, “o” signifies the AM articles observed to have an average segregation cell size within a range of 0.15 to 1.5 μm and judged as “Passed”, and “x” signifies the other AM articles, judged as “Failed”.

Based on the results of Experimental 2, it has been confirmed that in the selective laser melting step S2, the SLM process is preferably performed while controlling the thickness of the alloy powder bed h (unit: μm), the output power of the laser beam P (unit: W), and the scanning speed of the laser beam S (unit: mm/s) such that they satisfy the following formulas: “15<h<150” and “67×(P/S)−3.5<h<2222×(P/S)+13”. In other words, the hatched region is the passed region.

[Experimental 3]

(Manufacturing of Alloy Products IAP-1-1 to IAP-7-1 Formed of Powders IA-1 to IA-7 and Alloy Products CAP-1-1 to CAP-5-1 Formed of Powders CA-1 to CA-5)

An AM article (10 mm in diameter×50 mm in length) was formed of each of the alloy powders IA-1 to IA-7 and CA-1 to CA-5 prepared in Experimental 1 by the SLM process (the selective laser melting step S2). The thickness of each alloy powder bed h and the output power of the laser beam P were set at 100 μm and 100 W, respectively. The local heat input P/S (unit: W×s/mm=J/mm) was controlled by varying the scanning speed (mm/s) of the laser beam S so as to satisfy the passing conditions examined in Experimental 2.

Each AM article formed above was subjected to heat treatment at 500° C. with a holding duration of 4 hours (the internal strain relaxation annealing step S3) to manufacture Co based alloy products IAP-1-1 to IAP-7-1 formed of the powders IA-1 to IA-7 and Co based alloy products CAP-1-1 to CAP-5-1 formed of the powders CA-1 to CA-5.

(Microstructure Observation and Mechanical Properties Testing)

Test pieces for microstructure observation and mechanical properties testing were taken from the Co based alloy products LAP-1-1 to IAP-7-1 and CAP-1-1 to CAP-5-1 and subjected to microstructure observation and mechanical properties testing.

The microstructure observation was performed by SEM and through image analysis of SEM images thereof in a similar manner to Experimental 2 to measure the average size of segregation cells in the matrix phase crystal grains.

As to the mechanical properties testing, a creep test was conducted at 850° C. under a stress of 180 MPa to measure the creep rupture time. Based on the prescribed properties of high temperature members toward which the invention is directed, any creep rupture time of 1,500 hours or more was judged as “Passed”, and any creep rupture time of less than 1,500 hours was judged as “Failed”. Such creep properties can be deemed as comparable to those of Ni based alloy materials. In addition, a tensile test at a temperature of 800° C. was performed on parts of the inventive Co based alloy products to measure the high temperature tensile strength. When the high temperature tensile strength of 400 MPa or more is obtained, it can be deemed sufficiently high.

The results of Experimental 3 are shown in Table 2.

TABLE 2 Measurement and Testing Results of Alloy Products IAP-1-1 to IAP-7-1and CAP-1-1 to CAP-5-1. Average Size of Tensile Segregation Strength at Alloy Alloy Cells 800° C. Product No. Powder No. (μm) Creep Test (MPa) IAP-1-1 IA-1 0.55 Passed 500 IAP-2-1 IA-2 0.45 Passed 518 IAP-3-1 IA-3 1.5  Passed Not measured IAP-4-1 IA-4 0.65 Passed 498 IAP-5-1 IA-5 0.55 Passed 503 IAP-6-1 IA-6 0.15 Passed Not measured IAP-7-1 IA-7 0.80 Passed Not measured CAP-1-1 CA-1 0.55 Failed Not measured CAP-2-1 CA-2 2.8  Failed Not measured CAP-3-1 CA-3 2.5  Failed Not measured CAP-4-1 CA-4 120*⁾   Failed Not measured CAP-5-1 CA-5 100*⁾   Failed Not measured *⁾Average size of the matrix phase crystal grains is entered because the post-segregation cells were not formed.

As shown in Table 2, it is confirmed that the inventive alloy products IAP-1-1 to IAP-7-1 have microstructures in that the segregation cells are formed in the matrix phase crystal grains, and that the average size of segregation cells is within a range of 0.15 to 1.5 μm. Also, grains that seem to be the MC type carbide phase are dispersedly precipitated on parts of the boundary regions of segregation cells. It is recognized that all of these test pieces pass the mechanical properties.

On the other hand, the comparative alloy products CAP-1-1 to CAP-5-1 all failed the creep testing, although the segregation cells are formed in CAP-1-1 to CAP-3-1. To determine possible causes behind this, the comparative alloy products CAP-1-1 to CAP-5-1 were examined individually.

Regarding CAP-1-1, the excessive contents of C and Cr have resulted in excessive precipitation of Cr carbide grains. Regarding CAP-2-1, the excessive content of C and the excessive total content of “Ti+Zr+Hf+V+Nb+Ta” have resulted in coarsening of the segregation cells and an increased average size of the segregation cells. Regarding CAP-3-1, the excessive content of C, the insufficient content of N and the insufficient total content of “Ti+Zr+Hf+V+Nb+Ta” have resulted in excessive precipitation of the Cr carbide grains and an increased average size of the segregation cells. These results confirm that excessive precipitation of Cr carbide grains and/or an increased average size of the segregation cells lead to insufficient mechanical properties.

Regarding CAP-4-1 and CAP-5-1, insufficient total content of “Ti+Zr+Hf+V+Nb+Ta” (almost none) has resulted in no formation of the segregation cells in the matrix phase crystal grains of AM articles through the selective laser melting step S2. Therefore, it can be considered that no formation of the segregation cells and the MC type carbide phase grains in the matrix phase crystal grains has led to failure in the mechanical properties.

Based on the results of Experimental 3, it has been confirmed that IA-1 to IA-7, which have the chemical compositions specified in the invention, are desirable as starting materials for Co based alloy products. It has also been confirmed that formation of the segregation cells with an appropriate average size in the matrix phase crystal grains has led to the Co based alloy products having the creep properties comparable to or superior to those of the precipitation strengthened Ni based alloy materials. In addition, it has been confirmed that the inventive Co based alloy products exhibit very high tensile strength at high temperatures.

As a reason why the mechanical strength of the Co based alloy products is improved by forming the segregation cells, the following model can be considered, for example. As described before, the boundary regions (cell walls) of segregation cells are regions where the M component constituting an MC type carbide phase segregates. In other words, on the boundary regions of segregation cells, an element having an atomic radius larger than those of the Co, Cr and Ni components constituting the matrix phase is segregated. Because there is a clear difference in atomic radius between the inside regions and the walls of the segregation cells, it is considered that the cell walls of segregation cells (boundary regions) can act a role of the pinning sites for dislocation movement due to crystal deformations. This would contribute to the improvement of the mechanical strength.

The above-described embodiments and Examples have been specifically given in order to help with understanding on the present invention, but the invention is not limited to the described embodiments and Examples. For example, a part of an embodiment may be replaced by known art, or added with known art. That is, a part of an embodiment of the invention may be combined with known art and modified based on known art, as far as no departing from a technical concept of the invention.

LEGEND

-   100 . . . turbine stator blade; -   101 . . . inner ring side end wall; -   102 . . . blade part; -   103 . . . outer ring side end wall; -   200 . . . gas turbine; -   210 . . . compression part; -   220 . . . turbine part; -   221 . . . turbine nozzle; -   300 . . . heat exchanger; -   301 . . . separation layer; -   302 . . . fin layer; and -   303 . . . side bar portion. 

1. A product formed of a cobalt based alloy, the cobalt based alloy having a chemical composition comprising: 0.08 to 0.25 mass % of carbon; 0.1 mass % or less of boron; 10 to 30 mass % of chromium; 5 mass % or less of iron, 30 mass % or less of nickel, the total amount of the iron and the nickel being 30 mass % or less; tungsten and/or molybdenum, the total amount of the tungsten and the molybdenum being 5 to 12 mass %; 0.5 mass % or less of silicon; 0.5 mass % or less of manganese; 0.003 to 0.04 mass % of nitrogen; 0.5 to 2 mass % of an M component being a transition metal other than tungsten and molybdenum and having an atomic radius of more than 130 pm; and the balance being cobalt and impurities, the impurities including 0.5 mass % or less of aluminum, and 0.04 mass % or less of oxygen, wherein the product is a polycrystalline body of matrix phase crystal grains, and wherein in the matrix phase crystal grains of the polycrystalline body, segregation cells with an average size of 0.13 to 2 μm are formed, in which the M component is segregated in boundary regions of the segregation cells.
 2. The product according to claim 1, wherein the M component is a component being capable of generating an MC type carbide phase of a simple cubic crystal system, and wherein grains of the MC type carbide phase are precipitated on the boundary regions of the segregation cells.
 3. The product according to claim 1, wherein the M component of the chemical composition is at least one of titanium, zirconium, hafnium, vanadium, niobium and tantalum.
 4. The product according to claim 3, wherein: in the case that the chemical composition includes the titanium as the M component, content of the titanium is 0.01 to 1 mass %; in the case that the chemical composition includes the zirconium as the M component, content of the zirconium is 0.05 to 1.5 mass %; in the case that the chemical composition includes the hafnium as the M component, content of the hafnium is 0.01 to 0.5 mass %; in the case that the chemical composition includes the vanadium as the M component, content of the vanadium is 0.01 to 0.5 mass %; in the case that the chemical composition includes the niobium as the M component, content of the niobium is 0.02 to 1 mass %; and in the case that the chemical composition includes the tantalum as the M component, content of the tantalum is 0.05 to 1.5 mass %.
 5. The product according to claim 3, wherein the zirconium is an essential component as the M component of the chemical composition.
 6. The product according to claim 1, wherein the product exhibits a creep rupture time of 1,500 hours or more by a creep test under conditions of a temperature of 850° C. and a stress of 180 MPa.
 7. The product according to claim 1, wherein the product is a high temperature member.
 8. The product according to claim 7, wherein the high temperature member is a turbine stator blade, a turbine rotor blade, a turbine combustor nozzle, or a heat exchanger.
 9. The product according to claim 2, wherein the M component of the chemical composition is at least one of titanium, zirconium, hafnium, vanadium, niobium and tantalum.
 10. The product according to claim 9, wherein in the case that the chemical composition includes the titanium as the M component, content of the titanium is 0.01 to 1 mass %; in the case that the chemical composition includes the zirconium as the M component, content of the zirconium is 0.05 to 1.5 mass %; in the case that the chemical composition includes the hafnium as the M component, content of the hafnium is 0.01 to 0.5 mass %; in the case that the chemical composition includes the vanadium as the M component, content of the vanadium is 0.01 to 0.5 mass %; in the case that the chemical composition includes the niobium as the M component, content of the niobium is 0.02 to 1 mass %; and in the case that the chemical composition includes the tantalum as the M component, content of the tantalum is 0.05 to 1.5 mass %.
 11. The product according to claim 4, wherein the zirconium is an essential component as the M component of the chemical composition.
 12. The product according to claim 9, wherein the zirconium is an essential component as the M component of the chemical composition.
 13. The product according to claim 10, wherein the zirconium is an essential component as the M component of the chemical composition. 